Activity.get_preview fails for no window in canvas

When an activity deletes the window associated with the canvas, save may
fail because Gdk.cairo_create is given an argument of None.  Activity
continues to run without a window.

Reproduced with Chat activity and pressing Stop.

Avoid exception by returning None from get_preview when there is no
window.

src/sugar3/activity/activity.py

@@ -852,6 +852,9 @@ class Activity(Window, Gtk.Container):
             return None
 
         window = self.canvas.get_window()
+        if window is None:
+            return None
+
         alloc = self.canvas.get_allocation()
 
         dummy_cr = Gdk.cairo_create(window)